The substrate temperature is frequently an important parameter in thin film deposition and semiconductor processing operations. The substrate temperature and its uniformity can have a large effect on the quality and composition of the deposited layers. In molecular beam epitaxy for example, the substrate wafer is normally heated radiatively and rotated during the thin film growth operation. Physical contact between the wafer and a temperature sensor is not practical nor desirable because the sensor itself would cause local perturbations in temperature or even contamination of the substrate. Even if the wafer is not rotating, and heating is accomplished by thermal contact with a temperature regulated support, the temperature of the substrate can deviate substantially from the temperature of the support because of thermal contact problems which frequently exist in typical vacuum processing environments. Thus a non-contact method for measuring the temperature of the substrate is needed.
The simplest non-contact temperature measurement technique is to place a thermocouple close to the substrate so that it is in radiative contact with the substrate. While this solution is simple and cheap its accuracy is not adequate. In fact in molecular beam epitaxy it is not uncommon to have temperature errors of 100.degree. C. with this approach.
Optical pyrometry is another method for measuring the temperature of an object without touching it. However pyrometry has serious deficiencies for semiconductor processing applications. A pyrometer works by detecting the intensity of the thermal radiation that is emitted by any object that is not at the absolute zero of temperature (-273.degree. C.). The spectrum of the thermal emission depends on the product of the spectral dependence of the emissivity of the object and the emission spectrum of a black body at that temperature. For the temperature range of interest in semiconductor processing, namely between about 0.degree. C. and 1100.degree. C., the peak in the blackbody spectrum is in the infrared. However the emissivity of semiconductors is normally low in the infrared because semiconductors are transparent at long wavelengths. This means that the radiation that must be detected by the pyrometer is relatively weak which limits the temperature range of the technique for semiconductors to &gt;500.degree. C. for standard commercial pyrometers such as the instrument manufactured by IRCON. The transparency of semiconductors in the infrared also means care must be taken not to inadvertently measure the temperature of whatever is behind the semiconductor substrate, usually the heater. Yet another complication with pyrometers has to do with losses in optical elements used to transport the substrate radiation to the detector. In semiconductor processing operations it is not uncommon for optical elements such as windows and mirrors to become coated during the process. This affects the intensity of the thermal radiation from the substrate that reaches the detector which causes temperature errors. While the pyrometer can be useful for semiconductor temperature measurements it is not the complete answer.
It has been recognized for some time that the bandgap of a semiconductor is a reliable indicator of the temperature of the semiconductor because the bandgap is typically a smooth, almost linear function of temperature, in the 0.degree.-1000.degree. C. temperature range. Once the bandgap is known the temperature can be inferred from a one-time calibration for the particular material of interest. Various optical methods have been proposed for measuring the bandgap of the substrate. In the method of Hellman et al (J. Cryst. Growth 81, 38 (1987)) the radiation from heater filaments behind the substrate is transmitted through the substrate and detected by a detector outside the process chamber. By measuring the spectrum of the transmitted light they are able to infer the bandgap and hence the temperature. This method suffers from the variability in the intensity of the heater radiation as a function of the temperature of the heater. For example at low temperatures the heater produces very little radiation which makes accurate temperature measurements difficult.
To solve this problem Kirillov et al (U.S. Pat. No. 5,118,200) put a small lamp behind the substrate as an additional, brighter source of radiation. This increases the sensitivity of the measurement but introduces additional complications in the heater design. Because it is not practical to rapidly modulate the intensity of the light behind the substrate, this technique is not compatible with lock-in detection techniques which means that it is not possible to exclude background light from hot filaments or effusion ovens that may also be radiating in the same spectral range. In addition, with a fixed light source internal to the process chamber it is difficult to spatially resolve the temperature across the substrate. Temperature uniformity is a critical problem in growth of reproducible device structures with high yield.
These problems were solved by Weilmeier et al (Can. J. Phys. 69, 422 (1991)) who put the light source outside the process chamber and determined the bandgap from the spectrum of the back scattered light. In this method since the light source is outside the process chamber it does not interfere with the heater and is relatively easy to chop with a mechanical chopper. This makes lock-in detection techniques possible so that stray light from other sources can be rejected. To further enhance the sensitivity Weilmeier et al textured the back surface of the substrate and placed the detector in a non-specular position. The important optical signal in measuring the bandgap is the signal which is transmitted through the substrate. The diffuse reflection technique of Weilmeier et al detects only that part of the back scattered signal which has been transmitted through the substrate; the reflected signal from the front surface is specular and does not reach the detector which is located away from the specular reflection. This has the effect of eliminating the background signal reflected from the front surface of the substrate and thus reduces the sensitivity of the measurement to the surface properties of the substrate which are irrelevant as far as the temperature is concerned. A practical method for coupling the incident light into the growth chamber, and coupling the scattered light out of the growth chamber onto a photodetector using optical mirror ports and an optical fiber bundle has been demonstrated for the detection of scattered laser light by Lavoie et al (J.Vac.Sci. Technol. A10(4), 930 (1992)).
An elementary analysis method can be used to obtain a qualitative estimate of the bandgap from the diffuse reflection spectrum, for example by taking the wavelength where the diffuse reflectance is 50% of the peak value. Qualitatively the bandgap is at the wavelength where the diffusely scattered light intensity increases. However to determine the temperature accurately and reproducibly with a minimum of calibrations requires a precise procedure for finding an optical signature of the bandgap that can be related to the temperature. The point of inflection in the transmitted or reflected optical signal has been proposed by Kirillov et al as such an optical signature in the case of specular optical signals. The point of inflection measures a point on the absorption spectrum that lies below the optical bandgap. For maximum accuracy it is desirable to measure a point on the optical spectrum as close to the bandgap as possible. This is because the absorption below the bandgap is variable depending on the quality of the material and the doping density as is well known in the art. For accurate measurements of the absolute temperature it is desirable to have a technique which is as insensitive as possible to properties of the material that can vary between different specimens. The problems with absorption below the bandgap are exacerbated in the case of the diffuse reflection method where the back scattered signal experiences multiple reflections inside the substrate and hence has an effective long path inside the substrate. Also at high doping levels and high temperatures, or in small bandgap materials such as silicon, where the substrate is not transparent in the wavelength range below the bandgap, the point of inflection method is sensitive to the thickness of the substrate. This problem cannot simply be solved by calibration without first knowing the temperature dependence of the transmission of the substrate. The problem is particularly severe in the case of the diffuse reflection method where the internal optical path length can be large compared with the substrate thickness.